Dread wasting time? That’s a mistake (5)
I believe the answer is:
error
'that's a mistake' is the definition.
(synonyms)
'dread wasting time?' is the wordplay.
'dread' becomes 'terror' (synonyms).
'wasting' indicates named letters should be taken away.
'time?' becomes 't' (abbreviation).
'terror' with 't' taken out is 'ERROR'.
(Other definitions for error that I've seen before include "Trial and ___" , "Miscalculation" , "A mistake or inaccuracy" , "wrongdoing" , "Boob" .)